1. Field of the Invention
The present invention relates to methods and apparatus employing an ion implanter, particularly to methods and apparatus yielding electrostatically-controlled collimated scanning of a target using an ion beam in order to implant ions in the target. The present invention also relates to methods of measuring collimation, shaping the form of the ion beam and controlling its scanning, and to an ion implanter which is capable of implementing the aforementioned methods.
2. Discussion of the Related Art
A prior art ion implanter is shown in part in FIG. 11. In this ion implanter, ions extracted from an ion source (not shown in FIG. 11) are optionally analyzed by a magnetic field according to isotopic mass, accelerated to a desired energy level, and refocused at a spot in order to form an ion beam 2. The ion beam 2 is then deflected for electrostatically-controlled collimated scanning in an X-direction (e.g., horizontal direction X in FIG. 11) by the cooperation of two pairs of scanning electrodes 4 and 6 which are supplied with 180.degree. out-of-phase scanning voltages from a scanning power source 12. Thereafter, the deflected ion beam 2 illuminates a target 8 (e.g., a wafer) on a holder 10. In the prior art ion implanter of FIG. 11, the scanning power source 12 produces scanning voltages of +V and -V which each have a triangular waveform and which are out of phase by 180.degree..
The target 8 is mechanically scanned in a Y-direction (e.g., vertical direction Y in FIG. 11) which is substantially perpendicular to the X-direction by mechanically altering the position of the holder 10 and the target 8 through the use of, for example, a holder drive unit (not shown in FIG. 11). This mechanical scanning in the Y-direction, when coupled with the electrostatically-controlled collimated scanning in the X-direction achieved through the deflection of ion beam 2 by scanning electrode pairs 4 and 6, allows hybrid scanning in both the X-direction and the Y-direction to be performed. As a result of this hybrid scanning, ions can be implanted uniformly over the entire surface of the target 8.
In the prior art ion implanter of FIG. 11, a beam monitor 14 receives the ion beam 2 in order to measure a beam current I. The beam monitor 14 is provided at one end of a region where the ion beam 2 is scanned in the X-direction. The measured beam current I is fed to the Y-axis of a display 15 and a scanning voltage (e.g., V) is fed to the X-axis of the display so that the status of scanning performed by using the ion beam 2 is monitored during the ion implantation process. A waveform of the beam current I of scanning ion beam 2 is shown in FIG. 12.
In the prior art ion implanter described above, abnormal collimation of the ion beam 2 can occur for various reasons. Such reasons include the failure or deterioration of the scanning power source 12, the breakage or deterioration of the lead wires (not shown in FIG. 11) connecting the scanning power source 12 to the scanning electrodes 4 and 6, and unexpected variations in the configuration of the beam line. If abnormal collimation of the ion beam 2 occurs (i.e., if the ion beam 2 is not well collimated), the scanning speed of the ion beam 2 or the angle of its incidence on the target 8 fluctuates, thereby causing various problems such as lower uniformity in the ion implantation of target 8.
To detect and thereafter correct the problems caused by the abnormal collimation of ion beam 2, the operator of the prior art ion implanter must check for abnormal collimation of the ion beam based on the display of the waveform such as shown in FIG. 12. However, many years of experience are required to correctly interpret such a waveform display. Even if the operator is well experienced, for a variety of reasons, it is difficult for the operator to determine whether the collimation of the ion beam is abnormal by observing the waveform display. Such reasons include, but are not limited to, the following: (1) long-term variations in the collimation of ion beam 2 are difficult for the operator to locate; (2) even when the downstream scanning electrode 6 becomes completely ineffective, a waveform of beam current I, except for a slight shift in peak position, is identical to the one obtained under normal conditions; (3) depending on the ion species, the beam dose, the ion source and other factors, the waveform of beam current I may normally undergo variations or changes similar to those which occur as a result of abnormal collimation; and (4) even if abnormal collimation occurs, a waveform similar to the waveform for normal collimation is still sometimes obtained when the offset voltage of the scanning power source 12 is changed so as to shift the center of the beam scanning.
The scanning waveform controlling an ion beam conventionally has been shaped in accordance with either of the following two major techniques or methods:
(A) Using a multipoint beam monitor provided in the line of ion beam 2 or a single beam monitor which is movable in the scanning direction of the ion beam 2, the beam current is measured at several points along the scanning direction of the ion beam 2 and the scanning waveform controlling the ion beam 2 is shaped based on measured values of the beam current; or
(B) A scanning waveform of interest is generated using simulated calculations for distribution of potentials on the beam line of the ion beam 2.
However, methods (A) and (B) have the following problems. In method (A), if the beam monitor is not located in the same position as the target, the difference between the two positions (i.e., .DELTA.Z) causes the following inequality, with reference to FIG. 5, to result: EQU (X.sub.B '-X.sub.B :X.sub.A '-X.sub.A).noteq.(x.sub.B '-x.sub.B :x.sub.A '-x.sub.A).
This inequality introduces measurement inaccuracies and makes it impossible to create a desired waveform in a sufficiently correct way in order to achieve a speed of scanning over the target which is constant. Therefore, in order to avoid this problem, the beam monitor must be correctly positioned so that it is located in the same position as the target. As a result, a moving mechanism must be provided so that the beam monitor can be moved to a nonobtrusive position during ion implantation into the target. However, this increases the complexity of the shaping system.
Method (B) is disadvantageous in that it involves the construction of a complicated simulation model. Furthermore, it is particularly difficult to evaluate the effects of the edges of the scanning electrodes because such an evaluation requires considerable time to perform correct calculations. Moreover, even if data of preferred scanning waveforms are input into the scanning power source (e.g., arbitrary waveform generator 221 in the power source 22) in accordance with method (B), undesired factors such as the nonlinearity of high voltage amplifiers 222 and 223 can cause the power source 22 to produce an output scanning voltage which does not necessarily agree with the input scanning waveform data and which can vary from one measuring apparatus to another, thereby making it difficult to correctly achieve the shaping of scanning waveforms.